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Entropy 2018, 20(5), 375;

Transition of Transient Channel Flow with High Reynolds Number Ratios

Department of Mechanical Engineering, University of Sheffield, Sheffield S1 3JD, UK
Department of Maritime and Mechanical Engineering, Liverpool John Moores University, Liverpool L3 3AF, UK
Author to whom correspondence should be addressed.
Received: 24 March 2018 / Revised: 14 May 2018 / Accepted: 15 May 2018 / Published: 17 May 2018
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Large-eddy simulations of turbulent channel flow subjected to a step-like acceleration have been performed to investigate the effect of high Reynolds number ratios on the transient behaviour of turbulence. It is shown that the response of the flow exhibits the same fundamental characteristics described in He & Seddighi (J. Fluid Mech., vol. 715, 2013, pp. 60–102 and vol. 764, 2015, pp. 395–427)—a three-stage response resembling that of the bypass transition of boundary layer flows. The features of transition are seen to become more striking as the Re-ratio increases—the elongated streaks become stronger and longer, and the initial turbulent spot sites at the onset of transition become increasingly sparse. The critical Reynolds number of transition and the transition period Reynolds number for those cases are shown to deviate from the trends of He & Seddighi (2015). The high Re-ratio cases show double peaks in the transient response of streamwise fluctuation profiles shortly after the onset of transition. Conditionally-averaged turbulent statistics based on a λ_2-criterion are used to show that the two peaks in the fluctuation profiles are due to separate contributions of the active and inactive regions of turbulence generation. The peak closer to the wall is attributed to the generation of “new” turbulence in the active region, whereas the peak farther away from the wall is attributed to the elongated streaks in the inactive region. In the low Re-ratio cases, the peaks of these two regions are close to each other during the entire transient, resulting in a single peak in the domain-averaged profile. View Full-Text
Keywords: pipe flow boundary layer; turbulent transition; large eddy simulation; channel flow pipe flow boundary layer; turbulent transition; large eddy simulation; channel flow

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Mathur, A.; Seddighi, M.; He, S. Transition of Transient Channel Flow with High Reynolds Number Ratios. Entropy 2018, 20, 375.

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